Remote Invocation. 1. Introduction 2. Remote Method Invocation (RMI) 3. RMI Invocation Semantics

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1 Remote Invocation Nicola Dragoni Embedded Systems Engineering DTU Informatics 1. Introduction 2. Remote Method Invocation (RMI) 3. RMI Invocation Semantics

2 From the First Lecture (Architectural Models)... The architecture of a system is its structure in terms of separately specified components and their interrelationships. 4 fundamental building blocks (and 4 key questions): Communicating entities: what are the entities that are communicating in the distributed system? Communication paradigms: how do these entities communicate, or, more specifically, what communication paradigm is used? Roles and responsibilities: what (potentially changing) roles and responsibilities do these entities have in the overall architecture? Placement: how are these entities mapped on to the physical distributed infrastructure (i.e., what is their placement)? 2

3 Communication Paradigms 3 types: direct communication interprocess communication low level support for communication between processes in the distributed system, including message-passing primitives, socket programming, multicast communication remote invocation most common communication paradigm, based on a two-way exchange between communicating entities and resulting in the calling of a remote operation (procedure or method) indirect communication communication is indirect, through a third entity, allowing a strong degree of decoupling between senders and receivers. Examples: publish subscribe systems, distributed shared memory (DSM). 3

4 Remote Invocation RPC (Remote Procedure Call) the earliest programming model for distributed programming allows client programs to call procedures in server programs running in separate processes (and generally in different computers from the client). RMI (Remote Method Invocation) extension of local method invocation of object-oriented programming allows an object living in one process to invoke the methods of an object living in another process. Most famous example: Java RMI (see tutorial!) 4

5 Remote Method Invocation (RMI)

6 Let Us Start from Scratch: the Object Model (...in 2 slides...) An object-oriented program (Java, C++,...) consists of a collection of interacting objects, each of which consists of a set of data and a set of methods. An object can communicates with other objects by invoking their methods, generally passing arguments and receiving results (request/reply protocol). Objects can encapsulate their data and the code of their methods. Some languages (JAVA, C++) allow programmers to define objects whose instance variables can be accessed directly. BUT in a distributed object system, an object s data should be accessible only via its methods (or interface). 6

7 Actions in the Object Model An action in an object-oriented program is initiated by an object invoking a method in another object. The receiving object executes the appropriate method and then returns control to the invoking object, sometimes supplying a result. An invocation of a method can have 3 possible effects: the state of the receiver may be changed a new object may be instantiated (i.e., by using a constructor in Java) further invocations on methods in other objects may take place 7

8 How to extend the traditional object model to make it applicable to distributed systems?

9 The Distributed Object Model Each process contains a collection of objects some of which can receive both local and remote invocations whereas the other objects can receive only local invocations. Method invocations between objects in different processes, whether in the same computer or not, are known as remote method invocations. Method invocations between objects in the same process are local method invocations. 9

10 Remote Objects Remote objects: objects that can receive remote invocations. Fundamental concepts of the distributed object model: [Remote Object References] other objects can invoke the methods of a remote object if they have access to its remote object reference. [Remote Interfaces] every remote object has a remote interface that specifies which of its methods can be invoked remotely. 10

11 Remote Object Reference A remote object reference is an identifier that can be used throughout a distributed system to refer to a particular unique remote object. A remote object reference is passed in the invocation message to specify which object is to be invoked. Remote object references are analogous to local ones in that: the remote object to receive a remote method invocation is specified by the invoker as a remote object reference remote object references may be passed as arguments and results of remote method invocations 11

12 Remote Interface The remote interface specifies which methods of an object can be invoked remotely. The class of a remote object implements the methods of its remote interface. Objects in other processes can invoke only the methods that belong to the remote interface of a remote object. Local objects can invoke the methods in the remote interface as well as other methods implemented by a remote object. 12

Actions... in a Distributed Object System As in the non-distributed case: an action is initiated by a method invocation, which may result in further invocations on methods in other objects.

13 Actions... in a Distributed Object System As in the non-distributed case: an action is initiated by a method invocation, which may result in further invocations on methods in other objects. BUT in the distributed case: the objects involved in a chain of related invocations may be located in different processes or different computers. When an invocation crosses the boundary of a process or computer, RMI is used and the remote reference of the object must be available to the invoker. Remote object references may be obtained as the results of remote method invocations (example: A might obtain a remote reference to F from B) 13

Creation of Remote Objects When an action leads to the instantiation of a new object, that new object will normally live within the process where

14 Creation of Remote Objects When an action leads to the instantiation of a new object, that new object will normally live within the process where the instantiation is requested. If a newly instantiated object has a remote interface, it will be a remote object with a remote object reference. 14

15 Exceptions Any remote invocation may fail for reasons related to the invoked object being in a different process or computer from the invoker. Example: the process containing the remote object may have crashed or may be too busy to reply, or the invocation or result message may be lost. Remote method invocation should be able to raise exceptions! Timeouts that are due to distribution Exceptions raised during the execution of the method invoked: - attempt to read beyond the end of a file - attempt to access a file without the correct permissions

16 RMI Invocation Semantics

17 Local Method Invocation Semantics Local method invocations are executed exactly once exactly once invocation semantics = every method is executed exactly once This cannot always be the case for remote method invocation! Request-reply protocols, such as RMI, can be implemented in different ways to provide different delivery guarantees. These choices lead to a variety of possible semantics for the reliability of remote invocations as seen by the invoker. 17

18 Main Design Choices for Implementing RMI Retry request message: whether to retransmit the request message until either a reply is received or the server is assumed to have failed. Sender Receiver request deadlock! Error control mechanisms: timeout + retransmission of request msg 18

19 Main Design Choices for Implementing RMI Duplicate filtering: when retransmissions are used, whether to filter out duplicate requests at the server. msg Sender request Receiver reply msg timeout request duplication! Error control mechanisms: msg numbering scheme 19

20 Main Design Choices for Implementing RMI Retransmission of results: whether to keep a history of result messages to enable lost results to be retransmitted without re-executing the operations at the server. msg Sender request Receiver reply operation timeout request Error control mechanisms: operation numbering scheme + history of result msgs 20

21 Main Design Choices for Implementing RMI Combination of these choices lead to a variety of possible semantics for the reliability of remote invocations: Maybe, At-least-once, At-most-once. Duplicate filtering +/-? Retry request message Retransmission of results 21

22 RMI Invocation Semantics: Maybe The remote method may be executed once or not at all. Maybe semantics arises when no fault tolerance measures are applied. Useful only for applications in which occasional failed invocations are acceptable. This model can suffer from the following types of failure: omission failures if the invocation or result message is lost crash failures when the server containing the remote object fails. 22

23 Exercise Provide a CSP specification of a client-server system implementing a request-reply protocol compliant with the MAYBE semantics. 23

24 RMI Invocation Semantics: At-Least-Once The invoker receives either a result, in which case the invoker knows that the method was executed at least once, or an exception informing it that no result was received. Can be achieved by the retrasmission of request messages, masking the omission failures of the invocation or result message. This model can suffer from the following types of failure: crash failures when the server containing the remote object fails arbitrary failures, in cases when the invocation message is retransmitted, the remote object may receive it and execute the method more thn once, possibly causing wrong values to be stored or returned. 24

25 Exercise Provide a CSP specification of a client-server system implementing a request-reply protocol compliant with the AT-LEAST-ONCE semantics. 25

26 RMI Invocation Semantics: At-Most-Once The invoker receives either a result, in which case the invoker knows that the method was executed exactly once, or an exception informing it that no result was received, in which case the method will have been executed either once or not at all. Can be achieved by using a combination of fault tolerance measures (retransmission + duplicate filtering). The use of retries masks any omission failures of the invocation or result messages. Arbitrary failures are prevented by ensuring that for each RMI a method is never executed more than once. 26

27 Exercise Provide a CSP specification of a client-server system implementing a request-reply protocol compliant with the AT-MOST-ONCE semantics. 27

28 RMI Invocation Semantics Summary In Java RMI the invocation semantics is at-most-once. In CORBA is at-most-once but maybe semantics can be requested for methods that do not return results. 28

29 Remote Procedure Call (RPC)

30 RPC (... in one slide...) RPC (Remote Procedure Call): allows client programs to call procedures in server programs running in separate processes and generally in different computers from the client. CLIENT SERVER call rpc() function execute request call service return reply request completes service returns service executes 30

31 RPC vs RMI? A remote procedure call is very similar to a RMI in that a client program calls a procedure in another program running in a server process. Server may be clients of other servers to allow chains of RPCs. A server process must define in its service interface the procedures that are available for calling remotely. RPC, like RMI, may be implemented to have one of the choices of invocation semantics previously discussed (maybe, at-least-one, at-most-one). 31

Communication Paradigms

Communication Paradigms Nicola Dragoni Embedded Systems Engineering DTU Compute 1. Interprocess Communication Direct Communication: Sockets Indirect Communication: IP Multicast 2. High Level Communication